The present invention relates to an apparatus for removing gases from molten metal, especially from molten aluminum, in a ceramic melting receptacle or crucible which is accommodated in a furnace, between the detachable cover of which and the surface of the molten material there is produced a partial vacuum.
Apart from removing the gases, especially from molten aluminum, by chlorination or flushing with clorine-releasing agents or gases, from the flushing with inert gases, from the removal of gases by tapping, from the ladle degasification or ladle filtration degasification, from the removal of gases from the individual ingot, and from the circulation degasification, the important methods for removing gases from aluminum casting alloys in the foundry mold are the receptacle degasification and the ladle degasification. The last-mentioned method for removing gases provides for a melting receptacle or crucible which can be closed off by a cover; a bubble lance passes through the cover and extends to the bottom of the receptacle or crucible. A partial vacuum of less than 130 mbar is formed in the space between the cover and the surface of the molten metal, so that bubbles can form which rise in the molten material into the aforementioned space (Giesserei 66 (1979) Nr. 6, pages 56-62). German Patent 32 47 457 discloses a method for removing gases from molten metal, especially from molten aluminum, in a crucible having a cover, and also utilizing a partial pressure between the cover and the surface of the molten material. However, this method omits a bubble lance and uses a crucible having a porous wall through which gases which surround the receptacle enter the molten material and bubble therethrough. An absolute pressure of 50 to 500 mbar is produced between the cover and the surface of the molten material.
The important thing with apparatus where a relatively low partial vacuum is produced between the surface of the molten material and the cover of the melting receptacle or crucible is the tightness of the seal between the cover and the receptacle or crucible. Fluctuations of the partial pressure do not permit reliable removal of gases from the molten material. Even the smallest leaks at the space between the surface of the molten material and the cover allow a great variation of the vacuum to result in this space. Something that has to be taken into consideration in this connection is that the melting receptacle or crucible is generally made of ceramic material having an uneven outer surface, so that an absolute or nearly absolute tight seal can at best be achieved with a very complex arrangement or at high cost.
It is therefore an object of the present invention to provide a sealing arrangement for the space between the cover and the surface of the molten material in the receptacle or crucible, which sealing arrangement is extensively reliable and assures constancy of the partial pressure over a long period of time. At the required partial vacuum, generally in the order of magnitude of from 0.05 to 0.50 bar, no secondary air should enter directly from the outside into the aforementioned space between the surface of the molten material and the cover, thus preventing gas or air from passing through the molten material, be it by the utilization of a lance or on the basis of a porous crucible. It is a further object of the present invention that the means used to accomplish the above be structurally simple and easy to handle.